This specification relates to modifying web pages to decrease latency associated with retrieving the web pages.
Delivering web pages over the Internet allows computer users to receive search results, e-mail, news, games, entertainment, music, books, and web pages—all with a simple Internet connection. Web pages can include content such as text, graphics, audio, and computer scripts. Various content items included on a single web page are sometimes hosted by different Internet servers, and are retrieved when downloading the web page. In particular, a browser that retrieves a hypertext markup language (HTML) page and parses the page. Such pages commonly include elements that define numerous different features of the web page to be displayed to a user. Some of the features are defined within the HTML page, while other features are defined by specifying an address and/or other instructions for retrieving additional content. In the latter case, the browser makes a call to each specified address and/or in accordance with each set of instructions to retrieve the additional content, which may include images, JavaScript instructions, style sheets, and iframes (e.g., using an iframe tag that defines an inline frame which can be used to insert another web document). Web developers often use such features to incorporate previously defined content, to facilitate caching of styles or other resources (e.g., JavaScript, CSS style sheets and images) for use on related pages, and to avoid encoding all of the features of the web page into the HTML page itself.